1. Field of the Invention
The present invention relates to a voltage supply circuit for supplying voltage to a sensor such as a condenser microphone, and a microphone unit comprising the same.
2. Description of the Related Art
For voice communication in portable terminals, such as portable telephones, a technology using a microphone, called a condenser microphone, has been popularized. A condenser microphone is sometimes called a capacitor microphone or an electrostatic microphone. In a condenser microphone, one electrode of a capacitor is a diaphragm. This diaphragm detects the vibration of voice as a change of capacitance, and converts it into electric signals. A conventional microphone unit is disclosed in “PA acoustic system, (Kougakutosho Ltd., 1996)”
FIG. 1 shows a circuit of a condenser microphone unit 100 using a conventional condenser microphone. As FIG. 1 shows, the conventional condenser microphone unit comprises a condenser microphone 101, JFET (Junction Field Effect Transistor) 102, capacitor 103, resistors 104 and 105 and DC power supplies 106 and 108.
The condenser microphone 101 is a vibration sensor for generating output signals corresponding to the sound pressure of voice to be input. One electrode of the condenser microphone 101 is connected to the DC power supply 108 via the resistor 104, and the other electrode is grounded. A predetermined bias voltage is supplied by the DC power supply 108 to the condenser microphone 101. The output of the condenser microphone 101 is connected to the gate of the JFET 102. The JFET 102 is an amplification circuit for amplifying the output signals of the condenser microphone and generating the amplification signals. The amplification signals generated by the JEFT 102 are output via the output terminal 107.
In this condenser microphone unit, manufacturing dispersion occurs when the condenser microphone and the JFET are manufactured. This manufacturing dispersion appears as the dispersion of the inter-electrode distance of the capacitor and the dispersion of the amplification efficiency of the JFET. This manufacturing dispersion becomes a cause of the sensitivity dispersion of each condenser microphone unit.
A voltage supply circuit for supplying voltage to the sensor device, such as a condenser microphone, such that the sensor device can operate at an appropriate sensitivity even if manufacturing dispersion occurs, has been desired. Also a condenser microphone unit that can operate at an appropriate sensitivity according to the dispersion has also been desired.
To switch the sensitivity of the condenser microphone unit in a conventional condenser microphone unit, two condenser microphone units with different sensitivity settings are provided. The sensitivity is switched by switching the condenser microphone unit itself. With this configuration, however, a condenser microphone unit must be provided according to the levels of sensitivity to be switched. Therefore a condenser microphone unit that can select a plurality of sensitivities in one condenser microphone unit has been desired.